Refactor GroupOptions model and update documentation

- Removed the `available_nodes` field from the GroupOptions model in `group.py`.
- Updated imports in `pwd.py` by removing unused imports.
- Added a comprehensive roadmap document outlining the development stages and features for DiskNext Server.
- Created a detailed README for the models directory, documenting the database models and their relationships.

utils/password/pwd.py

@@ -4,7 +4,7 @@ from argon2 import PasswordHasher
 from argon2.exceptions import VerifyMismatchError
 from enum import StrEnum
 import pyotp
-from itsdangerous import URLSafeTimedSerializer, BadSignature, SignatureExpired
+from itsdangerous import URLSafeTimedSerializer
 from pydantic import BaseModel, Field
 
 from utils.JWT.JWT import SECRET_KEY